Why would the learning rate curve go backwards?
I have not used fastai library but this also happens on tensorboard when you have more than one training being recorded on the same plot.
Looking at the picture, I think this is a very special type of graph because for a single LR value you have 2 loss values associated. Put in other words, you have the same LR value for different loss values. My guess it that some time-dependent issue is messing things here.
Another intuition that may help to solve the issue is representing data differently. If you rotate the graph 90º left-wise you could see how the LR is evolving with different loss values. LR should be decreasing along with loss value, but in this case it is not like that either. So review how you are setting the LR too!
My list of to-check would be:

Check how you update LR according to the loss value
Check the LR recorder instance is only being used by one training at a time
Try plotting the value of LR (y) against your loss values (x)
Try plotting the value of LR (y) against the current epoch number (x)

Hope it helps